CMake: Don't use Windows path separators in install prefix

Sometimes CMake complains that the backslash is interpreted as unknown escape symbol.

CMakeLists.txt

@@ -17,6 +17,8 @@ if(NOT CMAKE_CONFIGURATION_TYPES)
     endif()
 endif()
 
+file(TO_CMAKE_PATH "${CMAKE_INSTALL_PREFIX}" CMAKE_INSTALL_PREFIX)
+
 # https://blog.kitware.com/cmake-and-the-default-build-type/
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
     set(default_build_type "Debug")